The natural lens of a human eye accommodates to change its focal length, which allows the person to focus on distant objects and near objects. When the eye focuses on a distant object, the lens power is at the low end of the accommodation range, which may be referred to as the “far” power. In contrast, when the eye focuses on a relatively close object, the lens power is at the high end of the accommodation range, which may be referred to as the “near” power. The accommodation range or add power is defined as the near power minus the far power.
The human eye contains a structure known as the capsular bag, which surrounds the crystalline lens. The capsular bag is transparent, and serves to hold the lens. In the natural eye, accommodation is initiated by the ciliary muscle and a series of zonular fibers, also known as zonules. The zonules are located in a relatively thick band mostly around the equator of the natural lens, and impart a largely radial force to the capsular bag that can alter the shape and/or the location of the natural lens and thereby change its effective power. The ciliary muscle applies radial forces largely to the peripheral edge of the natural lens. When the ciliary muscle contracts, the natural lens bulges slightly in the axial direction, producing more steeply curved anterior and/or posterior faces, and producing an increase in the power of the lens. When the ciliary muscle relaxes, radial forces are produced that decrease the optic power by flattening the lens.
A human eye can suffer diseases or conditions that impair or otherwise affect vision. For instance, a cataract may increase the opacity of the crystalline lens of the eye, causing loss of clarity or blindness. Presbyopia refers to the condition where the natural lens progressively loses its ability to accommodate and, therefore, to focus on near objects. Presbyopia occurs naturally with age, and it typically begins to noticeably affect vision at about the age of forty. In this regard, the accommodation range of a person less than ten years old could be eight diopters (or higher), while the accommodation range of a person more than fifty years old might only be two diopters (or less). Statistics indicate that the average accommodation range of a person forty years old is about four diopters, and that the accommodation range begins to get progressively worse after the age of forty.
Reading glasses are commonly used to counter the effects of presbyopia. However, reading glasses can be inconvenient and bothersome to many people. Existing or proposed surgical approaches include corneal modification, replacing the natural lens with an accommodating intraocular lens (IOL) or a multifocal IOL, and softening the crystalline lens using laser treatment. The existing and proposed surgical techniques, however, may not be very effective at improving the accommodation range and/or they may improve the accommodation range at the expense of visual clarity.